Clean up EPE::getStats()

* Move larger code blocks to separate methods to handle the
  requested stat block.

* Use the cb::const_char_buffer instead of copy it to a
  std::string

* Use early return instead of else if

Change-Id: I8c7d0de42bc452bc5a8b49b3fa9a0e3ee9bfcfce
Reviewed-on: http://review.couchbase.org/114594
Tested-by: Build Bot <[email protected]>
Reviewed-by: Jim Walker <[email protected]>

Author: trondn